The present invention relates to a method and a device for analysing landing system signals known as ILS "Instrument Landing System" signals. The device is intended in particular for the ground control of all the characteristics of instrument landing installations.
In French Patent Application FR-A-1 487 441, the Applicant has described a method of measuring an ILS signal implementing an analog technique evaluating the deviation of the aircraft with respect to the landing path, from the determination of the difference in modulation existing between the two components, 90 and 150 Hz, constituting the composite ILS signal.
The French Patent Application FR-A-2 596 547 shows a device for formulating radionavigation information relying, in one embodiment, on a digital architecture implementing a microprocessor assembly at the level of which the ILS signal is processed with an algorithm calling upon a recursive fast Fourier transform calculation.
These two requirements, which describe devices intended to serve as on-board navigation receivers, give rise to constraints distinct from that of a device for measurement and analysis; in particular, they necessitate integration of the flaws due, among other things, to the obstacles which may appear in the landing path, so as to prevent any tacking during approach. This "smoothing" of the information only allows small data renewal rates which, however, permit recourse to processing operations such as the recursive fast Fourier transform which demands long observation times.
By contrast, in an analysis device such as that of the invention, it is essential to be able to assess all the imperfections of the system to be controlled, this requiring particularly fast response times.
One purpose of the invention is to produce a measurement device having high performance, that is to say great accuracy and a not lower processing speed.
Another purpose of the invention is to produce a simple and compact device employing a restricted number of components and thereby permitting fast and reduced maintenance.
These purposes are achieved with an instrument landing system (ILS) signal analysis device including an analog/digital converter receiving the composite signal to be analysed and delivering a succession of values which can be processed in digital form, characterized in that the said signal to be analysed is furthermore applied to phase-locking means which deliver for the said analog/digital converter a sampling signal of frequency greater than the largest frequency of the components of the signal to be analysed, the digital processing of the values from the converter, carried out in real time between two sampling instants, allowing determination of the parameters for modulation of the ILS signal.
The phase-lock loop is synchronized directly with the frequency of the signal to be analysed, which frequency is extracted from a sub-harmonic of the 90 Hz and 150 Hz components constituting this signal.
This specific architecture allows the calculation procedure, implemented at the level of the digital processing assembly, to be synchronized directly with the composite ILS signal so as constantly to assess the maximum amplitude of the 90 and 150 Hz components which is contained in this ILS signal, and this despite the frequency or phase drift existing at the level of the signal or despite its distortion factor. Furthermore, the instantaneous processing between each sample of the numerical values acquired makes it possible to limit the memory capacity of the device and thus to afford it maximum compactness.
The digital processing of the values from the converter is carried out by a processing unit with which are associated a random-access memory, a non-volatile memory and an input/output module.
The non-volatile memory includes a first table in which are written numerical values corresponding to the value of a weighting window function for each sampling instant, as well as at least two tables in each of which are written numerical values representative, over a quarter period, of a cosine function of frequency equal to that of a characteristic component of the signal to be analysed.
Recourse to these tables of values avoids calculation, for each sample acquired, of the values of the functions employed in the calculation procedure.
Preferably, for the processing of the identification signal contained in the ILS signal, the analysis device according to the invention includes a digital filter using a weighting function whose equation is given by: ##EQU1## where T represents the-duration of observation of the signal to be analysed and k' and .alpha. are specified numerical coefficients.
The choice of this window (and in particular with k'=4 and .alpha.=0.54) makes it possible to obtain a filter having a very selective and quasi-linear frequency response in the pass-band.
As before, this weighting window is advantageously stored in a non-volatile memory in the form of a table of numerical values.
The device according to the invention is implemented in accordance with a method including the following steps:
a) acquisition of a sample S(nTe) of the ILS signal to be analysed by the analog/digital conversion of this signal, Te representing the sampling period and n being an integer number, the sampling frequency being greater than the largest of the frequencies k of the components of the signal to be analysed, PA1 b) multiplication of the value of the sample taken by a first numerical value corresponding to the value of a weighting window function at the acquisition instant: EQU S(nTe)=Fen(nTe).times.S(nTe) PA1 c) determination, at the frequencies k of the characteristic components of the ILS signal, of the real and imaginary parts of the spectrum of the sampled and weighted signal S(t): EQU R(k)=R(k)+S(nTe).times.cos 2.pi.k nTe PA1 d) repetition of steps a) to c) for all of the samples over a duration of observation T of the ILS signal, PA1 e) calculation of the modulation factors for the characteristic components of the ILS signal. ##EQU2## HF being the average value of S(nTe) calculated over all the samples taken. PA1 f) calculation of the difference (DDM) and sum (SDM) of the modulations (M (90) and M(150)) of the 90 and 150 Hz components of the ILS signal: EQU SDM=M(90)+M(150)and DDM=M(90)-M(150).
and EQU I(k)=I(k)+S(nTe).times.sin 2.pi.k nTe
By performing the above calculations on numerous samples, advantageously 540, the effect of the analog or quantization noise becomes virtually nil. Moreover, measurement of the amplitude of each component of the signal is effected selectively about each frequency, this having the effect of further limiting the wide band noise.
In a particular embodiment, it is possible to take just one sample of the signal to be analysed every second or third value from the tables of the cosine function, thereby making it possible to determine also the amplitude of the 2nd or 3rd harmonic components of this signal.
Other characteristics and advantages of the present invention will emerge better on reading the following description given, by way of non-limiting illustrative example, in connection with the attached drawings in which: